Are redox probes a useful indicator of film stability? An electrochemical,AFM and XPS study of electrografted amine films on carbon

The electrochemical response of solution-based redox probes is commonly used to detect and monitor the formation and stability of films grafted to conducting substrates. In this work we examine redox probe responses at films grafted to glassy carbon (GC) and pyrolysed photoresist film (PPF) by electrooxidation of aliphatic primary amines. Cyclic voltammograms are obtained before and after soaking the modified surfaces in phosphate buffer and acetonitrile. For conditions where films exhibit large changes in electrochemical blocking properties, AFM measurements of film thickness and surface roughness, and XPS measurements of surface composition, are used to monitor the stability of the films. No changes in film structure or composition can be detected, demonstrating that electrochemical loss of blocking properties cannot be equated with large-scale loss of surface film. The origin of changes in probe response is discussed.     

XPS and AFM characterization of aminosilanes with different numbers of bonding sites on a silicon wafer

The aim of this work was the preparation and characterization of a silicon surface modified by different self‐assembled aminopropylsilanes with the purpose of using them in sensor applications. Single‐crystal silicon wafers (111) were modified with aminosilanes that have different numbers of bonding sites: 3‐aminopropyltrimethoxysilane (APTMS), 3‐aminopropyldiethoxymethylsilane (APRDMS) and 3‐aminopropylethoxydimethylsilane (APREMS). We deposited the self‐assembled layers from a solution of aminosilanes in toluene under various reaction conditions. The surface composition and the chemical bonding were determined using X‐ray photoelectron spectroscopy. Furthermore, the surface morphology was investigated using atomic force microscopy. Our results show that the reactivity with the Si‐oxide layer and the polymerization of aminosilanes depend on the number of possible bonding sites. The APTMS reacted the most intensively with the Si‐oxide layer; a less intensive reaction was observed for the APRDMS; and the least intensive reaction was observed for the APREMS. Copyright © 2013 John Wiley & Sons, Ltd.     

XPS and AFM characterization of the enzyme glucose oxidase immobilized on SiO(2) surfaces

A process to immobilize the enzyme glucose oxidase on SiO2 surfaces for the realization of integrated microbiosensors was developed. The sample characterization was performed by monitoring, step by step, oxide activation, silanization, linker molecule (glutaraldehyde) deposition, and enzyme immobilization by means of XPS, AFM, and contact angle measurements. The control of the environment during the procedure, to prevent silane polymerization, and the use of oxide activation to obtain a uniform enzyme layer are issues of crucial importance. The correct protocol application gives a uniform layer of the linker molecule and the maximum sample surface coverage. This result is fundamental for maximizing the enzyme bonding sites on the sample surface and achieving the maximum surface coverage. Thin SiO2 layers thermally grown on a Si substrate were used. The XPS Si 2p signal of the substrate was monitored during immobilization. Such a signal is not completely shielded by the thin oxide layer and it is fully suppressed after the completion of the whole protocol. A power spectral density analysis on the AFM measurements showed the crucial role of both the oxide activation and the intermediate steps (silanization and linker molecule deposition) to obtain uniform immobilized enzyme coverage. Finally, enzymatic activity measurements confirmed the suitability of the optimized protocol.     

An AFM, XPS and wettability study of the surface heterogeneity of PS/PMMA-r-PMAA demixed thin films

A series of homopolymer/random copolymer blends was used to produce heterogeneous surfaces by demixing in thin films. The chosen homopolymer is polystyrene (PS) and the random copolymer is poly(methyl methacrylate)-r-poly(methacrylic acid) (PMMA-r-PMAA), whose acidic functions could be used as reactive sites in view of further surface functionalization. The proportion of each polymer at the interface was deduced from X-ray photoelectron spectroscopy (XPS) data using, on the one hand, the O/C ratio, and on the other hand, decomposition of the carbon peak of the blends in two components corresponding to the carbon peaks of PS and PMMA-r-PMAA. Combining the information from XPS with atomic force microscopy (AFM) images, water contact angle measurements and PS selective dissolution, it appears that the surfaces obtained from blends with a high PS content (90/10 to 70/30) display pits with a bottom made of PMMA-r-PMAA, randomly distributed in a PS matrix. On the other hand, the surfaces obtained from blends with a low PS content (30/70 to 10/90) display randomly distributed PS islands surrounded by a PMMA-r-PMAA matrix. The characteristics of the heterogeneous films are thought to be governed by the higher affinity of PMMA-r-PMAA for the solvent (dioxane), which leads to the elevation of the PS phase compared to the PMMA-r-PMAA phase, and to surface enrichment in PMMA-r-PMAA.     

Phase behavior and dewetting for polymer blend films studied by in situ AFM and XPS: from thin to ultrathin films

In this work, the film thickness (l0) effect on the phase and dewetting behaviors of the blend film of poly(methyl methacrylate)/poly(styrene-ran-acrylonitrile) (PMMA/SAN) has been studied by in situ atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The thinner film shows the more compatibility of the blend, and the phase separation of the film occurs at l0>5Rg (radius of gyration). An initially time-independent q*, the characteristic wavenumber of the phase image, which is in good agreement of Cahn's linearized theory for the early stage of spinodal decomposition, has been obtained in real space and discussed in detail. For 5Rg>l0>3Rg, a "pseudo-dewetting/(phase separation+wetting)" behavior occurs, where the pseudo-wetting is driven by the concentration fluctuation mechanism. For l0<3Rg, a "real dewetting/(phase separation+wetting)" behavior occurs.     

An electrochemical and XPS study of reduction of nitrophenyl films covalently grafted to planar carbon surfaces

Nitrophenyl (NP) films were grafted to glassy carbon and pyrolyzed photoresist films by electroreduction of the corresponding diazonium salt. The as-prepared, multilayered films were examined using electrochemistry and X-ray photoelectron spectroscopy (XPS). Electrochemical analysis confirmed the absence of electrooxidizable groups whereas XPS showed approximately 35% of N was present in a reduced form. The reduced N is assigned to azo groups, which are known to be electroinactive in the film environment. NP films were reduced electrochemically in three media and also by chemical reduction in ethanolic disodium sulfide. The concentrations of aminophenyl and hydroxylaminophenyl groups produced by each method were estimated electrochemically, and the relative amounts of unreacted NP groups were established from XPS measurements. Aminophenyl is the major product for all reduction methods, and Na2S gives the cleanest and most complete conversion to aminophenyl groups, with less than 5% residual NP. Reduced NP films were reacted with carboxylic acid and acid chloride derivatives; the highest yield of electroactive-coupled product was obtained for a film electroreduced in H2SO4 and reacted with acid chloride. The detailed electrochemical and XPS analysis reveals the limitations of electrochemistry for determining the composition of these films.     

XPS studies of thin polycyanurate films on silicon wafers

Thin films of a diandicyanato bisphenol A (DCBA) prepolymer on silicon substrates have been investigated. Angle dependent X-ray photoelectron spectroscopy reveals some thickness-dependent features, which lead to an adsorption model for the DCBA prepolymer molecules. The adsorption of the first layer is governed by the interaction of the triazine rings with the substrate surface.     

XPS studies of thin polycyanurate films on silicon wafers

Thin films of a diandicyanato bisphenol A (DCBA) prepolymer on silicon substrates have been investigated. Angle dependent X-ray photoelectron spectroscopy reveals some thickness-dependent features, which lead to an adsorption model for the DCBA prepolymer molecules. The adsorption of the first layer is governed by the interaction of the triazine rings with the substrate surface.     

XPS investigations of thin tantalum films on a silicon surface

Ultra thin tantalum-based diffusion barriers are of great interest in copper metallisation technology. Even the smallest amounts of copper that diffuse into the active silicon regions on a microprocessor will alter their semiconducting properties thus leading to failure of the device. In the present work Ta films were deposited on silicon by electron beam evaporation and magnetron sputtering. The background of this study is investigation of interface formation, which is expected to have substantial influence on the properties of thin Ta films. All experiments were carried out under UHV conditions. This was necessary because Ta is a very reactive metal and is readily oxidized even at low oxygen partial pressure. The Ta4f peak, as a sensitive indicator of the chemical state, was analysed and compared to that for standard samples. Silicide formation is assumed to occur at the Ta/Si interface.     

Combined XPS and AFM study of cluster-containing polymers based on Rh

Rh₆- monomer and polymer-immobilized complexes have been characterized using XPS and AFM. Polymer-immobilized clusters were obtained by the reaction of Rh₆(CO)₁₅CH₃CN with copolymer of allyldiphenylphosphine and styrene. AFM study shows the change of surface morphology of the above copolymers. XPS data demonstrated the change of charge state of Rh atoms under monosubstitution of the CO-group for Rh₆- monomer complexes as well as in copolymer cluster complexe after the catalysis process of hydrogenation.     

Multilayer nitroazobenzene films covalently attached to carbon. An AFM and electrochemical study

Nitroazobenzene films have been grafted to pyrolyzed photoresist films by electrochemical reduction of the corresponding diazonium salt in acetonitrile solution. Two component films were also prepared by electrochemically grafting methylbenzene layers to preformed NAB films. Voltammetric investigation of the films in aqueous acid medium and the measurement of film thickness using atomic force microscopy (AFM) lead to new insights into film structure. In aqueous acid solution, the azobenzene groups have no detectable electroactivity and not all nitro groups in the films can be reduced. These findings point to a compact film structure in which proton diffusion is limited. There may also be spatial inhibition of the conformational changes that accompany azobenzene reduction. For increasingly thick NAB films, the peak for reduction of the nitro groups moves to more negative potentials and the peaks become more asymmetric in shape. These changes are interpreted in terms of the dielectric properties and the rate of proton diffusion in the films. Film thickness was measured by ploughing through the film with an AFM tip. When an NAB film prepared in acetonitrile solution is reduced in aqueous acid, the film thickness decreases by more than 50%. The changes can be partially reversed by treatment in acetonitrile-electrolyte solution and hence are attributed to ion-solvent induced swelling and shrinking. Thus, the large decrease in thickness detected by AFM after treatment of the film in aqueous acid is consistent with the compact film structure revealed by electrochemistry.     

AFM of adsorbed polyelectrolytes on cellulose I surfaces spin-coated on silicon wafers

Thin layers of cellulose I nanocrystals were spin-coated onto silicon wafers to give a flat model cellulose surface. A mild heat treatment was required to stabilize the cellulose layer. Interactions of this surface with polyelectrolyte layers and multilayers were probed by atomic force microscopy in water and dilute salt solutions. Deflection–distance curves for standard silicon nitride tips were measured for silicon, cellulose-coated silicon, and for polyelectrolytes adsorbed on the cellulose surface. Transfer of polymer to the tip was checked by running deflection–distance curves against clean silicon. Deflection–distance curves were relatively insensitive to adsorbed polyelectrolyte, but salt addition caused transfer of cationic polyelectrolyte to the tip, and swelling of the polyelectrolyte multilayers.     

Identification of mobile species in cationic polymer lubricant layer on silicon oxide from AFM and XPS analyses

The nanoscale spreading of a cationic polymer lubricant (CPL) film consisting of polydimethylsiloxane with quaternary ammonium salt side chains on a SiO(2) surface was studied with the disjoining pressure measurements using atomic force microscopy. CPL shows a monotonic decrease in disjoining pressure as the film thickness increases from 1.3 to 4.5 nm, which suggests stable spreading in this thickness range. Comparing the spreading rates calculated from disjoining pressure and the viscosity of CLP to the self-healing time after tribo-contacts revealed that the ionic form may not be the main mobile species. The X-ray photoelectron spectroscopy analysis found that the CPL film on SiO(2) has about 30% of the quaternary ammonium salts (cationic groups) reduced to tertiary amines (neutral groups). The reduced CPL polymer has much lower viscosity than the original CPL polymer and yields a spreading rate consistent with that measured at the macroscale. Thus, the mobile component in the CPL/SiO(2) film responsible for self-healing is concluded to be the reduced tertiary amine components of CPL.     

In situ AFM study of amelogenin assembly and disassembly dynamics on charged surfaces provides insights on matrix protein self-assembly

Because self-assembly of matrix proteins is a key step in hard tissue mineralization, developing an understanding of the assembly pathways and underlying mechanisms is likely to be important for successful hard tissue engineering. While many studies of matrix protein assembly have been performed on bulk solutions, in vivo these proteins are likely to be in contact with charged biological surfaces composed of lipids, proteins, or minerals. Here we report the results of an in situ atomic force microscopy (AFM) study of self-assembly by amelogenin--the principal protein of the extracellular matrix in developing enamel--in contact with two different charged substrates: hydrophilic negatively charged bare mica and positively charged 3-aminopropyl triethoxysilane (APS) silanized mica. First we demonstrate an AFM-based protocol for determining the size of both amelogenin monomers and oligomers. Using this protocol, we find that, although amelogenin exists primarily as ~26 nm in diameter nanospheres in bulk solution at a pH of 8.0 studied by dynamic light scattering, it behaves dramatically differently upon interacting with charged substrates at the same pH and exhibits complex substrate-dependent assembly pathways and dynamics. On positively charged APS-treated mica surfaces, amelogenin forms a relatively uniform population of decameric oligomers, which then transform into two main populations: higher-order assemblies of oligomers and amelogenin monomers, while on negatively charged bare mica surfaces, it forms a film of monomers that exhibits tip-induced desorption and patterning. The present study represents a successful attempt to identify the size of amelogenin oligomers and to directly monitor assembly and disassembly dynamics on surfaces. The findings have implications for amelogenin-controlled calcium phosphate mineralization in vitro and may offer new insights into in vivo self-assembly of matrix proteins as well as their control over hard tissue formation.     

Advances on structuring, integration and magnetic characterization of molecular nanomagnets on surfaces and devices

This critical review represents a concise revision of the different experimental approaches so far followed for the structuration of molecular nanomagnets on surfaces, since the first reports on the field more than ten years ago. Afterwards, a presentation of the different experimental approaches followed for their integration in sensors is described. Such work involves mainly two families of sensors and devices, microSQUIDs sensors and three-terminal devices for single-molecule detection. Finally the last section is devoted to a detailed revision of the different experimental techniques that can be used for the magnetic characterization of these systems on surfaces, ranging from magnetic circular dichroism to magnetic force microscopy. The use of these techniques to characterize other nanostructured magnetic materials, such as nanoparticles, is also revised. The aim is to give a broad overview of the last advances achieved with these techniques and their potential and evolution over the next years.     

XPS spectra of chromium monosilicides and disilicides obtained by in situ fractured clean surfaces

X-ray photoelectron spectroscopy spectra of chromium monosilicide (CrSi) and disilicide (CrSi₂) were collected from a clean surface prepared by fracturing the bulk silicide compound in a spectrometer under ultrahigh vacuum; the analytical procedure for the phase identification of the Cr–Si system was examined. A negligible binding energy shift was observed in the Cr 2p_3/2 level between elemental Cr and CrSi₂, whereas the energy of CrSi was 0.2 eV lower than that of Cr and CrSi₂. The satellite peak in the Cr 2p spectra originating from the plasmon-loss phenomena was found only for CrSi and CrSi₂. The binding energy of Si 2p shifted, reflecting the silicide phases; the energy of CrSi₂ and CrSi was 0.3 eV higher and lower, respectively, than that of elemental Si. Although a slight difference in the spectral shape was observed in the valence band region, the phase identification was considered unreliable. However, the energy shifts of Si 2p and the presence of the plasmon-loss peak in the Cr 2p spectra provide important insights into the phase identification of the Cr–Si system.     

TAT peptide immobilization on gold surfaces: a comparison study with a thiolated peptide and alkylthiols using AFM, XPS, and FT-IRRAS

A TAT peptide was used to functionalize a gold surface by three different methods: adsorption from solution, microcontact printing, and dip-pen nanolithography (DPN). The composition and structure of the modified gold was characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Fourier transform -infrared reflection absorption spectroscopy (FT-IRRAS). We used two well-studied alkylthiols, mercaptohexadecanoic acid and 1-octadecanethiol, as a comparison in order to understand the structure of the TAT peptide monolayers prepared by the three methods. AFM studies allowed us to assess the homogeneity after each modification protocol. XPS was used to characterize the chemical composition of the gold surface after each functionalization procedure. The XPS results showed that surfaces modified with the TAT peptide by the three methods exhibit similar surface chemistry. Finally, FT-IRRAS experiments allowed us to conclude that the structure of the alkyl chains of the TAT peptides is fairly disordered and different after each procedure. Regardless of the type of surface functionalization method used, the monolayer of TAT peptide formed on the surface was of "liquidlike" nature.     

Obtaining thin films of “reactive polymers” on metal surfaces by electrochemical polymerization: Part II. Alcohol substituted polyphenylene oxide films

The selective electrochemical oxidation of the phenol function in the case of hydroxymethyl phenol derivatives (o^?, m^?, p-hydroxybenzyl alcohol) leads to “reactive polymer” films of polyphenylene oxide substituted by CH₂OH groups. The transformation of the hydroxyl function into an ester function by acetyl chloride indicates the reactivity of the CH₂OH group. As for the family of carbonylated polyphenylene oxide films, reactivity is limited to the superficial layers of film. Average film thickness is between 50–100 nm; however with the ferrocene-ferricinium system acting as a redox catalyst, it can reach about 300 nm. This catalytic mechanism intervenes only when the oxidation potential of the ferrocene-ferricinium couple is very similar to that of the phenol derivative.     

An atomic force microscope (AFM) and tapping mode AFM study of the solvent-induced crystallization of polycarbonate thin films

Thin films of bisphenol-A polycarbonate (PC) were progressively crystallized in a controllable manner under the action of an appropriate plasticizing agent. No conducting layer was applied to the polymer surface, so that imaging with the atomic force microscope (AFM) left the sample exposed for additional plasticizer treatment. The PC sample was observed many times throughout the crystallization process using the AFM in both the contact and tapping modes. Data regarding the growth process for spherulites in addition to high-resolution morphology studies were achieved. © 1996 John Wiley & Sons, Inc.     

UV-vis-infrared optical and AFM study of spin-cast chitosan films

Optical properties of spin-cast chitosan films have been determined in the infrared, visible, and ultraviolet region of the spectrum using spectroscopic ellipsometry. Optical constants for the UV–vis–near IR spectra from 130 to 1700 nm were determined using Cauchy dispersion forms combined with Lorentzian oscillator models in the absorptive shorter wavelength regions. Infrared index of refraction and extinction coefficients from 750 to 4000 cm⁻¹ were determined using ellipsometric data fits to dispersion models based on harmonic oscillators. This modeling determined that optical anisotropy was present and measurable over all wavelength regions of ellipsometric data.

To obtain information on the micro- and nano-scale surface structure, tapping mode atomic force microscopy (AFM) imaging was employed to determine morphology and roughness information of dry spin-cast chitosan films.